1. Field of the Invention
The present invention relates to a vacuum extraction system for creating a vacuum in a reactor vessel in which a chemical vapor deposition (CVD) process is carried out to form a thin film on a substrate for production of, for example, a semiconductor, and also relates to a trapping device incorporated in such a vacuum extraction system for treating a starting gas pulled from the reactor vessel thereinto.
2. Description of the Related Art
In a CVD process such as a thermal decomposition CVD process, a photo-assisted CVD process, and a plasma-assisted CVD process, a starting gas is decomposed and activated within a reactor vessel to produce active species as decomposition products, and the active species are deposited on a substrate to form a thin film thereon. For example, when a silane gas (SiH.sub.4, Si.sub.2 H.sub.6) is used as the starting gas, a silicon thin film is formed on the substrate. During the CVD process, the starting gas is continuously introduced into the reactor vessel, but a vacuum is continuously drawn from the reactor vessel through a vacuum extraction system connected to thereto, so that a given low pressure is maintained in the reactor vessel to properly control the growth of the thin film.
As is well known, in the CVD process as mentioned above, the starting gas cannot be completely used in the formation of the thin film, and a large part thereof is pulled from the reactor vessel into the vacuum extraction system, so that a mechanical booster pump, a rotary pump, and other elements included therein may be damaged by the starting gas pulled from the reactor vessel. Namely, a deposition component (e.g., Si) of the starting gas pulled from the reactor vessel is separated therefrom, and a build-up of the separated deposition component occurs in an interior of the vacuum extraction system. Especially, the build-up of the separated deposition component on internal movable parts of the pumps is problematic because of a high expense thereof. For this reason, the pumps of the vacuum extraction system are connected to the reactor vessel through the intermediary of a water-cooled trap for removing the deposition component from the starting gas, whereby the build-up of the deposition component of the starting gas with respect to the internal movable parts of the pumps can be prevented.
Recently, CVD reactor vessels have become larger for the production of semiconductors at low cost, so that an amount of the starting gas used has also increased. This means that a large amount of the starting gas must be treated in the trap of the vacuum extraction system. Nevertheless, the conventional water-cooled trap fails to treat the large amount of the starting gas due to a low throughput capacity thereof. Namely, a part of the starting gas passes through the trap, and reaches the pumps, to thereby cause a premature deterioration of the pumps.